1. Field of the Invention
This invention generally relates to an optical information recording and reproducing apparatus and in particular to an optical pick-up device for use in such an optical information recording and reproducing apparatus. More specifically, the present invention relates to a control system for controlling the operation of such an optical pick-up device.
2. Description of the Prior Art
An optical disc recording and reproducing apparatus using an optical disc as a medium for storing information is drawing much attention in the recent years. Such an optical disc recording and reproducing apparatus typically includes an optical disc, which is rotatably supported and driven to rotate in a predetermined direction, an optical pick-up device, which is provided to be movable relative to the optical disc in the radial direction of the optical disc as well as in the direction perpendicular to the recording surface of the optical disc, and means for moving the optical pick-up device relative to the optical disc so as to carry out focusing and tracking controls. In such an optical disc, a recording track having a track width in the order of 1 micron is formed on its data recording surface at a track pitch in the order of 2 microns. And, a laser beam having a spot size in the order of 1 micron is projected onto the recording track of the optical disc. With the optical disc in rotation, in a recording mode, the light intensity of the laser beam applied to the optical disc is modulated in accordance with the data to be recorded on the optical disc to thereby have the data recorded on the optical disc. On the other hand, in a reading mode, the variation of the light intensity of the light reflecting from the recording surface of the optical disc is detected to reproduce the stored data.
As described above, the optical pick-up device is moved relative to the optical disc so as to cause the laser spot to track the recording track of the optical disc and to cause the laser spot to have a predetermined size at the recording surface of the optical disc. For this purpose, the optical pick-up device produces focusing and tracking error signals for carrying out a servo control of the optical pick-up device as well as a data reproduction signal for reproducing the recorded data. The overall structure of a typical optical pick-up device is illustrated in FIG. 7a. In the illustrated example, the focusing action of a laser beam is carried out using the knife edge method. Described more in detail, a laser beam emitted from a laser diode or semiconductor laser device 1 is collimated by a coupling lens 2 and the thus collimated laser beam passes through a polarization beam splitter 3 and then through a quarter wavelength plate 4 where the laser beam is polarized. Then, the laser beam passing through the quarter wavelength plate 4 passes through an objective lens 5 to be focused onto a recording track defined on the recording surface of an optical disc 6.
The light reflecting from the optical disc 6 again passes through the objective lens 5, this time in the reversed direction, and then through the quarter wavelength plate 4 where the plane of polarization is varied. As a result, the light entering into the beam splitter 3 after reflection from the optical disc 6 is reflected upwardly toward a lens 7. After passing through the lens 7, a half of the light is reflected by a prism 8 which also defines a knife edge, and, as shown in FIG. 7b, the light reflected by the prism 8 focuses upon a tracking servo control light receiving device 9 having a pair of light receiving surfaces arranged in the tracking direction T. On the other hand, the remaining half of the light focused on a focusing servo control light receiving device 10 having a pair of light receiving surfaces arranged side-by-side with respect to a straight line in parallel with the knife edge of the prism 8. That is, the pair of light receiving surfaces of the light receiving device 10 is arranged in the direction of advancement of the recording track of the optical disc 6.
Although not shown specifically in FIG. 7a, the optical pick-up device also includes a tracking mechanism for positioning the objective lens 5 relative to the optical disc 6 in the radial direction and also a focusing mechanism for positioning the objective lens 5 relative to the optical disc 6 in a direction perpendicular to the recording surface of the optical disc 6. In the present specification, for the sake of convenience, the term "objective lens moving mechanism" will be used as a mechanism which is a combination of both of tracking and focusing mechanisms.
Then, with the help of a servo control unit which is not shown, based on a difference between output signals from the two light receiving surfaces 9a and 9b of the light receiving device 9, a positional (tracking) error of the laser beam on the surface of the optical disc 6 is detected. And, the tracking mechanism is controlled in accordance with such a tracking error to move the objective lens 5 so as to keep the positional discrepancy between a recording track of the optical disc 6 and the projected laser beam as small as possible. On the other hand, a focusing error signal of the laser beam is detected in the following manner. When the projected laser beam is properly focused on the optical disc 6, the position of convergence of the laser beam passing through the lens 7 agrees with the position of demarcation between the pair of light receiving surfaces 10a and 10b of the light receiving device 10, so that each of the pair of light receiving surfaces 10a and 10b receives the same amount of light.
When the optical disc 6 is shifted further away from the objective lens 5 as shown in FIG. 8b, the position of convergence of the laser beam from the lens 7 is located between the lens 7 and the light receiving device 10, so that the light receiving surface 10a receives more light than the light receiving surface 10b. On the other hand, when the optical disc 6 is shifted closer to the objective lens 5 as shown in FIG. 8c, the position of convergence of the laser beam from the lens 7 is located behind the light receiving device 10, so that the light receiving surface 10a receives less light than the light receiving surface 10b. As a result, based on a difference in the amount of light received by the pair of light receiving surfaces 10a and 10b, there may be produced a focusing error signal. And, the focusing mechanism is controlled so as to minimize such a focusing error by means of the servo control unit which is not shown, whereby the objective lens 5 is caused to move so as to minimize the focus error of the laser beam impinging upon the recording track of the optical disc 6. In addition, output signals from the pair of light receiving surfaces 9a and 9b are combined with output signals from the pair of light receiving surfaces 10a and 10b to reproduce data recorded on the optical disc 6.
The recording track on the optical disc 6 is segmented into sectors in order to carry out recording and/or reproducing of data efficiently. As shown in FIG. 9a, one such sector includes an identification signal region ID, in which a sector address or the like for indicating its identity is provided, a flag region FLG for storing a flag indicating the status of its sector, such as presence of recorded data or defective sector, and a data region DATA for storing data for this sector. And, the flag region FLG and the data region DATA are separated from each other by a gap GAP. Sectors are arranged along the recording track TR such that they are separated from one another by a gap GAP between the two. Typically, the recording track TR is a continuous groove formed on the recording surface of the optical disc 6 in the form of a spiral, and such a track is typically formed by using a stamper. However, during the manufacture of such an optical disc 6, depending on the condition of the stamper, there is produced a variation or scatter in the depth of the recording track TR and such a scatter can be a cause for varying the efficiency of reflection locally.
If such a variation in the efficiency of reflection is present in the gap region GAP at the beginning or end of the data region DATA, the following problem could happen at the time of recording data. That is, during recording of data, the light intensity of a laser beam is increased nearly to ten times of that at the time of reproduction of data in order to vary the efficiency of reflection of the recording medium. Thus, when the light intensity of the laser beam is changed to the recording level at the beginning of the data region DATA, a laser spot SP of the laser beam extends between the gap GAP and the data region DATA as shown in FIG. 10a. Accordingly, the leading portion A of the laser spot SP is located in the data region DATA and the remaining portion B of the laser spot SP is located in the gap GAP.
On the other hand, the light reflecting from the portion A of the laser spot SP impinges upon the light receiving surface 10a of the light receiving device 10 and the light reflecting from the portion B of the laser spot SP impinges upon the light receiving surface 10b. Thus, as described above, depending on a difference between the output signals from the respective light receiving surfaces 10a and 10b, there is produced a focusing error signal as shown in FIG. 9b. For this reason, if the rate of reflection is lower at the gap GAP than at the data region DATA, the light receiving surface 10a receives more light than the light receiving surface 10b at the time of initiation of recording data, so that the level of the focusing error signal becomes significantly large. As a result, the focusing error means erroneously determines that the focusing error has increased to thereby move the focusing mechanism though the in-focus condition is, in fact, present. In this case, malfunctioning in focusing control takes place. A similar erroneous focusing operation takes place due to the similar reason in the case where the rate of reflection is decreased at the gap GAP at the end of the data region DATA.
In some of the optical pick-up devices, the dividing line between the pair of light receiving surfaces 9a and 9b extends in parallel with the edge of the beam splitting prism 8 similarly with the light receiving device 10, and in such a case, the malfunctioning of tracking error control can take place.